Wide range dimmable fluorescent lamp ballast system

ABSTRACT

The present invention and its related methods of operation provide a wide range dimmable high frequency lamp ballast system having a resonant circuit associated with each lamp or lamp set for providing a resonant frequency which is substantially higher than the drive frequency at which the lamp or lamp set is driven, and means for automatically selecting a drive frequency to be a submultiple of the resonant frequency of the resonant circuit, the selection of a submultiple being dependent on lamp operating conditions.

This is a continuation of application Ser. No. 07/501,538, filed Mar.30, 1990.

BACKGROUND OF INVENTION

The function of a fluorescent lamp ballast is to connect the lamp bulbsto the power line in a way that will provide just enough current to thebulbs to operate them at the desired brightness level. Since the bulbshave a negative resistance characteristic which is dependent on beamcurrent, temperature, bulb type and age, and is very different duringstarting than during operation once started, this is not a simple task.Standard magnetic ballasts perform the current control function byinserting a relatively large iron core inductor between the bulbs andthe source of power. This achieves the control function with adequatestability but to change the brightness level requires changing theinductance, which cannot be done in a simple manner based on a controlknob position or control D.C. voltage. Electronic ballasts simulate theaction of the inductor with switchmode regulator circuits using feedbacktechniques to control the voltage across the bulbs based on measuredcurrent through them. Over a wide brightness range the impedance of thebulbs changes so much that instability usually results, and the bulbsflicker, turn off, or get into an inefficient operating state where thespecial coating on the filaments quickly burns off, making the bulbsunusable.

Some current electronic ballasts deal with this problem by operating thebulbs at a relatively high frequency (above the normal audio range) anduse inductors and capacitors to form a resonant circuit around thebulbs, with a resonant frequency at or near the frequency at which thebulbs are driven. Thus, as the bulb impedance changes, the "Q" of theresonant circuit changes with it, since the bulb is part of the resonantcircuit, and more or less drive voltage is immediately available asrequired, even though the feedback control circuit might not be able torespond quickly enough if the resonant circuit were not there. Thistechnique works well over a limited range of brightness, but to applythis technique over a wide brightness range, substantial changes in theinductance and capacitance forming the resonant circuit would berequired, and there is no practical way to accomplish this.

The present invention uses a resonant circuit around each bulb or bulbset in a similar fashion, but instead of using a resonant frequency ator near the drive frequency, a resonant frequency much higher than thedrive frequency is used, i.e. 130 KHz., and the drive frequency isselected to be an exact submultiple of the resonant frequency of theinductive and capacitive resonant circuit components around the bulb.Different submultiple frequencies are selected under different operatingconditions, achieving many benefits which would be obtained by changingthe inductance and capacitance values of the resonant circuit around thebulb if such changes were practical, which they are not.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to operatefluorescent lamps over a wide range of brightness while maintainingstable and efficient performance.

It is another object of the invention to allow cooler operation of thefluorescent lamps by not requiring current flow through the lampfilaments.

It is yet another object of the invention to provide around eachfluorescent bulb set a resonant circuit, but instead of using a resonantfrequency at or near the drive frequency, a resonant frequency muchhigher than the drive frequency is used.

It is yet still another object of the invention to provide means forselecting the drive frequency to be an exact submultiple of the resonantfrequency of the inductive and capacitive resonant circuit componentsprovided in circuit around each fluorescent bulb or bulb set.

It is still yet another object of the invention to provide for theautomatic selection of different submultiple frequencies under differentoperating conditions, thereby achieving many benefits which would beobtained by changing the inductance and capacitance values of theresonant circuit connected around each bulb or bulb set if such changeswere practical, which they are not.

It is another object of the invention to provide an energy efficientwide range dimmable fluorescent lamp ballast system which automaticallyswitches between a first (original) lamp drive frequency and a selectedsubmultiple frequency, or between two submultiple frequencies, orbetween three or more different submultiple frequencies, or even tooperate at a single submultiple frequency, and maintain stable andefficient lamp operation and performance.

Many other objects and advantages of the present invention will bereadily apparent from the following detailed description of theinventive system operation and its methods of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram illustrative of the system of thepresent invention.

FIG. 2 is a diagram showing an alternate lamp/ballast connection schemefor a very low brightness application.

FIG. 3 is a schematic diagram of the frequency control circuit of Block4 broadly shown in FIG. 1.

FIG. 4 is a schematic diagram of the switch element drive circuits ofBlock 3 broadly shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, block 1 is an EMI (Electro-MagneticInterference) filter, using standard technology to prevent interferencefrom the switching regulator circuits 2 from feeding onto the powerline.

Block 2 illustratively represents a combination of standard switch moderegulator circuits to rectify energy from the power line, temporarilystore the energy in capacitors and inductors, and output power tocapacitors C21 and C22 and the switching elements SW1 and SW2 inresponse to the difference between the brightness control input BC and afeedback signal from transformer T4, diodes D1 through D4, and resistorR1, such feedback signal being provided to Block 2 via the feedbackcircuit FBC. The brightness control input BC may be derived from apotentiometer, not shown, or from an external input.

Block 3 illustratively represents the circuits for driving the lampballast drive signal switching elements SW1 and SW2, which are arrangedso that SW1 is conducting and SW2 is nonconducting for one half cycle,and then SW1 becomes nonconducting and SW2 conducting for the next halfcycle. This procedure is repeated cycle after cycle at a lamp ballastdrive signal frequency determined by Block 4. A square wave power signalis thus produced at the junction of SW1 and SW2, with respect to thecommon point at the junction of C21 and C22. This lamp ballast drivepower signal is supplied to bulbs B1 through B6, through the resonantcircuits respectively consisting of inductors L1 through L3 andcapacitors C1 through C6, along with the small additional capacitancerepresented by the dotted line connected to ground at the right handedge of FIG. 1. Note that capacitor C23 provides a low impedance path athigh frequencies between the common point, referenced above, and ground.

B1 through B6 are fluorescent bulbs (lamps), each bulb having afilament, connected to two pins P, on each end. Ordinary ballasts,including most electronic ballasts, require separate connections to eachfilament pin to enable heating current to flow through each filament, inaddition to the beam current which flows through the filaments on itsway to the plasma path through the gases inside the bulb. With thisinventive ballast configuration, only the beam current flows through thefilaments, so that the two pins on each end of the bulb can be connectedthrough a single wire S.

One advantage of the invention is to allow operation of a number ofbulbs with only one set of switching elements (shown as SW1, SW2). Toaccomplish this, the lighting ballast circuit is configured intochannels. Three channels are shown: the first consists of bulbs B1 andB2, along with L1, C1, C4, T1, C7, Q1, R2, and C10. The second channelconsists of bulbs B3 and B4, with L2, C2, C5, T2, C8, Q2, R3, and C11.The third channel consists of bulbs B5 and B6, with L3, C3, C6, T3, C9,Q3, R4, and C12. More than three channels may be used, connected insimilar fashion, or only one or two channels may be used.

Each of the channels described above includes a resonant circuit whosefrequency is determined by the inductor L1, L2, or L3, and a respectivecapacitance consisting of C1, C2, or C3, along with the relatively smallcapacitance of the respective bulbs to the common point set forth abovethrough C4, C5, or C6 and C23. Capacitors C4, C5, and C6 have relativelylarge capacitance and act as nearly zero impedance connections to thebulbs at the frequencies of operation. Their purpose is to solve apotential problem well known in the technology of fluorescent lampballasts which is that the bulbs have a tendency to conduct beam currentmore readily in one direction than the other, and combined with theirnegative resistance characteristic, this can easily result in conductionin one direction only. However, with capacitors C4, C5, and C6 in thelighting circuit channels, such a tendency to favor conduction in onedirection over the other is quickly compensated by a buildup of D.C.voltage across the capacitors which is just sufficient to equalize thecurrent flow in the two directions.

The before-mentioned inductive and capacitive components in the resonantcircuit of each channel are manufactured to be essentially identical toeach other, so that the resonant frequency of each channel will beessentially identical. The resonant frequencies will change withconditions such as temperature or aging, and possibly otherenvironmental effects, but each should change in the same fashion and inabout the same amount.

Transformers T1, T2, and T3, and similar additional transformers (ifused), perform the function of forcing beam current to be sharedapproximately equally between the channels, even though bulb impedancesmay not match exactly. A separate transformer for each channel may beused, as shown, or the windings shown may all be wound on a common core,so that a single transformer can perform the sharing function for allchannels. If only a single channel is used, no sharing transformer isrequired. In that case, the connection from the bottom of the last bulbin the single channel would go through a single winding on T4 and thendirectly to the common point connecting to the junction of capacitorsC21 and C22.

In the case of a single channel, L1, C1, C4, T4, C7, Q1, R2, C10, C21,C22, and C23 would remain connected as shown, but the other componentslisted in the above description would not be required.

It is to be noted in FIG. 1 that two bulbs are shown connected in seriesinto each channel. Actually, more than two bulbs or as many bulbs asdesired, may be connected in series into each channel, provided thatsufficient voltage is available to operate them and the breakdownvoltage ratings of the switching elements and other components will notbe exceeded. Alternatively, only a single bulb in each channel is alsoacceptable. Another alternative lamp (bulb) connection configuration forvery low brightness is shown in FIG. 2, which is simply exemplary sinceother configurations could be utilized.

Transformer T4 acts as a current transformer to monitor the averagecurrent through all the lamps. Each channel connects through a singleturn primary winding W of T4, inducing a current in the secondarywinding equal to the sum of the currents in each channel divided by theturns ratio. This current is rectified by diodes D1 through D4, andflows through R1, producing a D.C. voltage proportional to the averagebulb current. This D.C. voltage is compared to the brightness controlinput by the regulator circuitry of Block 2. This regulator controls theD.C. output voltage provided across capacitors C21 and C22 to produce anaverage lamp (bulb) current proportional to the desired brightness. Amore detailed description of the Block 2 circuitry will be presentedbelow.

The heart of the operation of the invention is accomplished in Block 5,in conjunction with that of block 4. The amplitude and phase measurementcircuitry of Block 5 has two separate, but related, functions. Itreceives its inputs from the junctions of L1 and C1, L2 and C2, and L3and C3 via lines 7, 8, and 9, and C7, C8 and C9 respectively. Thesethree input points allow the voltages driving the three lamp channels tobe monitored with respect to the common point connected from thejunction of C21 and C22 via line 10 to Block 5. With a square wave lampdrive signal being applied to each resonant circuit, as set forth above,and having a frequency which is a submultiple of the resonant frequencyof the resonant circuits, some amplitude of signal at the resonantfrequency will be harmonically excited by the square wave drive signaland will be present at the input monitor points at the junctions ofL1-C1, etc. If the bulb impedance is low, the "Q" at the resonantfrequency will be low, and the resonant frequency signal at themonitoring point will be low. Conversely, if the bulb impedance is high,then the "Q" and the resonant frequency signal at the monitoring pointwill be high. If the components making up the resonant circuits areproperly selected, and the square wave lamp drive frequency is selectedat the proper submultiple of the resonant frequency, then over a verywide range of beam current, representing an equally wide range of lampbrightness, the resonant frequency signal at the monitoring point willfall into one of two categories:

(1) If the lamp bulb(s), which is (are) part of the resonant circuitbeing monitored, is (are) turned on and operating efficiently, themonitored signal will be at a relatively low level, over the entire beamcurrent range.

(2) If the lamp bulb(s), which is (are) part of the resonant circuitbeing monitored, is (are) not conducting or is (are) in an inefficientand partially conducting state, then the monitored signal will be at arelatively high level.

If, at a particular moment, the state of conduction of the bulb(s)causes the monitored signal level to be at an intermediate value betweenthese two conditions, then the very nature of the resonant circuits,driven at the proper submultiple of their resonant frequency, willrapidly cause the bulb to become more or less conductive until itreverts to one or the other of the two categories just described above.

In the preferred embodiment of the present invention depicted in theFIGS. 1, 3 and 4, the resonant frequency of the resonant circuits is setat 130 KHz, the normal square wave lamp drive frequency is set at 26 KHzand a start frequency of 43.33 KHz is utilized. The 26 KHz normaloperation lamp drive frequency signal is the 5th submultiple of the 130KHz resonant frequency, and the start frequency is the 3rd submultipleof 130 KHz. If the bulb condition is in category (1) the outputfrequency of Block 4 will switch to a higher submultiple number such asthe 7th, 9th, 11th, etc. for increased efficiency once the lamp hasstarted. If the bulb condition is in category (2), the output frequencyof Block 4 will switch to a lower submultiple number such as 1 or 3 toallow for more power delivery to the bulb. The present invention hasbeen designed to automatically select the odd submultiples of theresonant frequency, but the present invention should not be limited tothis design since the invention system can be modified within its scopeto select even submultiples of the resonant frequency. Also, forexample, the lamps could be driven: at a start frequency of 130 KHz (the1st submultiple) for a dim condition; at an operating submultiple of 3for medium brightness; or at an operating submultiple of 5 for maximumbrightness.

As shown in FIG. 1 the monitored signal referenced above is applied toeither Q1, Q2, or Q3, through a voltage divider consisting of either C7,R2, and C10; C8, R3, and C11; or C9, R4, and C12; and the combination ofC13, R5, D5, and D6. R6, R7 and C14 form a bias network controlling thesensitivity of the transistors Q1, Q2, Q3 and minimizing dependence ontemperature.

If the bulb condition is in category (1), operating efficiently, thecorresponding transistor, either Q1, Q2, or Q3, will be off. If the bulbcondition is in category (2), off or operating inefficiently, thecorresponding transistor will turn on. These three transistors areconnected in an "open collector" logic mode, so that if any one of themturns on, a low level logic signal will be sent to Block 4's RANGESELECTION input via line RS. If or when such low level logic signal issent to the frequency control circuit 4, the output frequency of Block 4will switch to a lower submultiple (higher frequency) of the resonantfrequency of the resonant circuits. This will result in a great increasein power delivery to any bulb which is in the category (2) state, andunless the bulb(s) is (are) defective or not connected into the circuitproperly, will cause the bulb(s) to quickly switch to and operate withinthe category (1) state. Assuming the bulb(s) successfully switch(es) tothe category (1) state, the monitored signal(s) will then drop inamplitude below the threshold required for turning on the transistor(Q1, Q2, or Q3), and Block 4's output frequency will return to theoriginal (normal operation) submultiple frequency.

As will be explained more fully later, the frequency control circuitmeans 4 may include a timer circuit so that if it stays in the higherfrequency (lower submultiple) condition for more than a short time, anassumption is made that a bulb is either defective or not connectedproperly, and in either of these cases the drive power to all the lampsis shut off for a substantial period of time, to be restarted later, andif the defect has been cleared, normal operation will resume. If thedefect remains, power shutoff will recur.

One may now wonder whether it may be better to always operate in thelower submultiple (higher frequency) mode, rather than to automaticallyselect a higher or lower submultiple dependent upon lamp conditions. Theanswer to that question is that operating in the lower submultiple modedoes put much more power into a bulb which is off or in an inefficientpartially conducting state, but it cannot put nearly as much power intoa bulb, once it has fully turned on, as operating in the highersubmultiple mode. And, whatever power level is being put out, operationis more efficient at the higher submultiple mode once the lamp isstarted and conducting properly.

The remainder of the circuitry shown in FIG. 1 for Block 5 acts as anautomatic fine tuning phase and frequency control circuit means. Theactual resonant frequency of the various inductors and capacitors aroundthe bulbs may vary as explained above. If block 4 alone produced anominal frequency equal to the proper submultiple of the expectedresonant frequency, it would not necessarily be accurate. And besidesthe potential variations in the actual resonant frequency, the circuitryof block 4 may not produce the exact frequency desired, because of thetolerances of its components or other effects. Therefore, the presentinvention includes provisions for verifying and correcting the matchbetween the submultiple frequency, produced by Block 4, and the resonantfrequency of the resonant circuits around the bulbs. Actually, suchresonant frequency is the approximate average of the individual resonantfrequencies of the resonant circuits in the one or more channelsimplemented. This is accomplished as follows: The signal at the drainterminal of FET transistor Q4 is a composite signal representing suchaverage, for the various channels, of the resonant frequency signals atthe monitored points referred to hereinabove. It has been noted earlierthat C13, R5, D5 and D6 form part of a voltage divider, which scales themonitored signals at the resonant frequency to the proper amplitude foroperating transistors Q1, Q2, and Q3. C13 and R5, in conjunction withthe remainder of that voltage divider, are chosen to provide animpedance which represents the correct phase of the resonant frequencysignal for comparison with the square wave drive signal to verify thefrequency match or to indicate the amount and direction of mismatch. D5and D6 limit the maximum amplitude of the signal at the drain terminalof Q4 so that the automatic frequency correction signal being producedby Block 5 will be limited in range to be able to correct for smallvariations in frequency, but will not be able to pull the frequency outof the nominal range.

Transistors Q4 and Q5 and their associated components form a sample andhold circuit. Block 4 provides a square wave signal at the drivefrequency currently being used, which is differentiated by C20 and R10via circuit line into short negative and positive going pulses. R11 andC16 filter out any noise spikes that are present. The negative goingpulses charge C15 through D8 to provide a negative bias voltage throughR9, keeping Q4 and Q5 off most of the time. However, each positive goingpulse momentarily turns Q4 and Q5 on through D7, charging C17 towhatever the current voltage is at Q4's drain terminal. C17 remains atthat voltage until the next positive going pulse, when it is updated bythe same process. Thus, the voltage across C17 is a D.C. voltageproportional to the instantaneous voltage at the resonant frequency atthe time a positive going edge of the square wave drive signal occurs,and represents the phase of the resonant frequency signal. If the drivefrequency is lower than the exact submultiple frequency of the actualinstantaneous resonant frequency, the voltage across C17 will benegative, whereas if the drive frequency is too high, the voltage acrossC17 will be positive. If the drive frequency is exactly right, thevoltage across C17 will be zero. This voltage is filtered by R12 and C18and buffered by Q6, with C19 filtering out any residual noise spikes,and then applied to the FINE TUNING input of block 4 via circuit line12. This fine tuning signal acts as a feedback control signal to causeBlock 4 to adjust its frequency output to compensate for changes orinaccuracies in either the resonant circuit components around the bulbsor in the circuitry of block 4 itself to insure a correct submultipledrive frequency.

An alternative circuit configuration for D7 is an ordinary diodecomponent reversed in polarity with respect to the polarity connectionof D7 in FIG. 1.

In the present embodiment of the invention, the frequency controlcircuit shown in Block 4 of FIG. 1 and in FIG. 3 is implemented using anindustry standard TL493 pulse width modulated control integrated circuitU1. The FINE TUNING input referred to above is used as a voltage sourcefor the frequency determining resistor R14. This FINE TUNING input,because of D5 and D6, as mentioned above, has a limited amplitude andcan only effect the frequency over a limited range.

The RANGE SELECTION input RS to Block 4 from Block 5 is used to connectan additional frequency determining resistor R15 in parallel with theone going to the FINE TUNING input, to change the frequency range up toa higher frequency (lower submultiple). While this range selectionresistor R15 is active, the automatic frequency control means discussedabove continues to operate in the same manner to correct the frequencyto its new higher value, and when the range selection resistor R15 isinactive, the Block 4 circuit automatically reverts to its originalsubmultiple or frequency mode of operation. A detailed description ofthe circuitry of Block 4 will now be presented with reference to FIG. 3.

Integrated Circuit U1 is an industry standard TL493 or equivalent pulsewidth modulation control circuit. Most of its functions are part of theregulation circuitry of Block 2, where it is used in accordance withnormal industry practice. The remainder of the functions of U1 areutilized for Block 4. U1's clock rate is controlled by externalcomponents connected to the "CT" and "RT" terminals. These componentsare shown in FIG. 3 and their functions are explained hereinbelow.

Capacitor C24 is connected between the "CT" terminal and common, and itsvalue is selected to provide the desired frequencies in conjunction withthe values of R14 and R15 as described below. The signal at the "CT"terminal of U1 is also supplied as a frequency output to Block 3 tocontrol the switching frequency of the lamp drive circuitry.

The FINE TUNING input from block 5 via line FT is more or less positivewith respect to common, depending on the direction and amount offrequency correction needed. Resistor R14 supplies the correct amount ofcurrent into the "RT" terminal to control the frequency to the desiredvalue.

The RANGE SELECTION input from block 5 via line RS goes through resistorR17 to the non-inverting (+) input of Integrated Circuit U2, acomparator. Capacitor C25 filters any noise present which may causeunintended operation of the frequency range selection. The inverting (-)input of U2 connects to a bias voltage set between the two levels of theRANGE SELECTION input. This bias voltage may be derived from a resistivevoltage divider (not shown) connected between the B+ supply and common.For example, such voltage divider could include two resistors, thejunction of which connects to the inverting (-) input of U2, thenon-junction end of one resistor connecting to B+, and the non-junctionend of the other resistor connecting to common.

Resistor R16 provides positive feedback to insure that the output of U2will always be either fully on (negative) or fully off (positive). WhenU2 is off, diode D9 is non-conducting, so that the operation of U1 isnot affected by the RANGE SELECTION input. But, when the RANGE SELECTIONinput becomes active (low), U2's output goes low, and diode D9 suppliesadditional current through Resistor R15 to raise the frequency generatedby U1 to the desired submultiple value.

Resistor R18 connects from the output of U2 to the input of IntegratedCircuit U3, a non-inverting buffer.

Capacitor C26 provides a delay allowing U2's output to go negative for ashort time without effecting the output of U3. However, if U2's outputstays negative for a longer time, as it will if a lamp is defective ornot properly connected, then U3's output will switch low, latching U3'sinput until C26 has discharged, after which normal operation willresume.

While U3's output is low, U4, an inverting buffer, supplies a high levelshutoff output to block 3 via line 14, to shut off power for the lampsuntil U3 switches back high again. A detailed description of thecircuitry of Block 3 will now be presented with reference to FIG. 4.

As shown in FIG. 4, Integrated circuit U5 is a CMOS D type flip flop,type 4013 or equivalent. The frequency input from Block 4 via line 13 isa ramp signal which goes positive at a relatively slow rate and thengoes negative very rapidly. Resistor R19 keeps transistor Q7 turned on(with its collector low) most of the time. However, when the frequencyinput signal goes low, capacitor C28 supplies enough negative current toovercome the normal positive bias current through R19 and momentarilyturn Q7 off, allowing resistor R20 to pull the clock input of U5 up andclock the flip flop.

So long as the shutoff input via line 14 from block 4 is low, its normalstate, each clock pulse changes the state of the flip flop outputs. TheQ and Q bar outputs will switch, one being high while the other is low,and then vice versa. The connection from the Q bar output to the D inputcauses this state change with each clock pulse. If the shutoff inputgoes high, both the set and reset inputs to U5 go active, causing boththe Q and Q bar outputs to go high and stay high as long as the shutoffinput remains high. As shown in FIG. 4, the Q output is also connectedto C20 of Block 5 of FIG. 1 via circuit line 11.

Q8 and Q9 are P Channel FET transistors, with their source terminalsconnected to the B+ supply, their gate terminals connected to U5's Q barand Q outputs, respectively, and their drain terminals connected to thecenter tapped primary of transformer T5, whose center tap connects tocommon. During normal operation, transistors Q8 and Q9 are alternatelyturned on and off, one being on while the other is off and vice versa.This produces square wave drive signals across each of the two secondarywindings of T5, which alternately turn the two before-mentionedswitching elements SW1 and SW2 of FIGS. 1 and 4 on and off. These switchelements are shown as N channel FET transistors, but could beimplemented as other types of switching elements. The secondary windingsof T5 are isolated and can be equipped with the proper number of turnsto drive whatever kind of switching elements are required.

When the shutoff input mentioned above causes U5's Q and Q bar outputsto both go high, transistors Q8 and Q9 are both turned off, and no driveis applied to transformer T5, causing both switching elements SW1 andSW2 to remain off, thereby removing the normal square wave drive to theoutput section of the circuitry and shutting off the current to thefluorescent bulbs B1-B6.

With additional reference to alternative lamp (bulb) configurations forvery low brightness, as shown by example in FIG. 2, at very lowbrightness levels, the beam current of the lamp(s) becomes so low thatefficient current transfer from the filaments to and from the plasmapath through the lamp cannot take place without extra heating of thefilaments. The invention includes a provision for providing extracurrent through the filaments at very low beam current levels ifoperation down to very low brightness levels is required. FIG. 2 showsan example of this option for one channel, wherein the filament pins oneach end of each bulb connect to an auxiliary transformer T5, as shown,rather than being tied together and connecting directly to the ballast.The primary winding of T5 has a large number of turns compared with eachof the secondary windings, and connects through capacitor C23, whosevalue is selected to be resonant with the primary inductance of T5 atapproximately the same frequency as the resonant frequency of theresonant circuits referred to earlier. Thus, at very low beam currentlevels, when the impedance of the bulb or lamp is high, there is a highlevel of signal at the resonant frequency, which produces substantialcurrent flow through T5 and each filament winding. At higher brightnesslevels, the bulb impedance is lower and there is less signal at theresonant frequency, so there will be less filament current. At highbrightness levels, where efficiency is most important, filament currentwill be almost zero, so that negligible power will be wasted. Eachchannel is intended to use a separate transformer connected in the samemanner as T5, if plural channels are used.

With further reference to FIGS. 1 and 4, one pair of switch elements SW1and SW2 are shown as a "half-bridge" type. The present invention appliesequally well to other switch circuit arrangements, such as a full bridgetype with four switching elements in an "H bridge" arrangement, wherethe load would be connected between the halves of the "H bridge".

The present invention is intended to cover any fluorescent lamp ballastsystem where the lamps are associated with resonant circuits and thedrive for the lamps is supplied at a frequency at or near one or moresubmultiple frequencies of the resonant frequency. One of thesubmultiple frequencies may be the resonant frequency itself i.e. 1stsubmultiple.

As set forth above the invention provides a system and methods forswitching back and forth between at least two submultiple frequencies,but the invention is intended to cover the options of always operatingat a single submultiple or of operating at three or more differentsubmultiples.

With the present invention system and methods fluorescent lamps can beoperated at much higher brightness levels than known to be available inthe field of this invention. For example ordinary 40 watt fluorescentlamps can be operated at power levels of 80 watts or more.

While the invention system has been described in conjunction with aspecific preferred embodiment thereof, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended to embrace all such alternatives, modifications andvariations which fall within the spirit and scope of the appendedclaims.

I claim:
 1. In a wide range dimmable high frequency fluorescent lampballast system of the type having associated with each lamp or lamp seta resonant circuit having a resonant frequency at or near the drivefrequency at which said lamp or lamp set is driven, the improvementcomprising: resonant circuit means for providing a resonant frequencywhich is substantially higher in frequency than the drive frequency atwhich a lamp or lamp set is driven, and means for automaticallyselecting said drive frequency to be a submultiple of the resonantfrequency of said resonant circuit means, dependent upon lampconditions, whereby different submultiple frequency drive signals areselected with respect to different lamp operating conditions.
 2. A widerange dimmable high frequency lamp ballast system comprising incombination:lamp means having associated therewith resonant circuitmeans for providing a resonant frequency which is substantially higherin frequency than the drive frequency at which said lamp means isdriven; and means for automatically selecting, dependent upon theoperating condition of the said lamp means, said drive frequency to be asubmultiple of the resonant frequency of said resonant circuit means,whereby different submultiple frequency drive signals are selected withrespect to different lamp operating conditions.
 3. A system as definedin claim 2, further comprising:means for producing a submultiple lampdrive power signal which is selected in accordance with a detected lampoperation condition; and means for detecting lamp operation conditionsand dependent therefrom providing control signals which control saidmeans for producing said submultiple lamp drive power signal toselectively produce a submultiple of said resonant frequency inaccordance with said detected lamp operation condition.
 4. A system asdefined in claim 3, further comprising:amplitude and phase measurementcircuit means for detecting the amplitude, phase and frequency of theresonant frequency signal of said resonant circuit means, said amplitudebeing representative of a lamp operating condition.
 5. A method ofoperating a wide range dimmable high frequency lamp ballast systemcomprising the steps of:providing a resonant circuit means for each lampor lamp set means of said system; providing a resonant frequency of saidresonant circuit which is substantially higher in frequency value thanthe drive frequency at which said lamp or lamp set means is driven; andautomatically selecting, dependent upon the operating condition of thesaid lamp means, said drive frequency to be a submultiple of saidresonant frequency of said resonant circuit, whereby differentsubmultiple frequency drive signals are selected with respect todifferent lamp operating conditions.
 6. A method as defined in claim 5,further comprising the steps of:producing a submultiple lamp drive powersignal which is selected in accordance with a detected lamp operationcondition; and detecting lamp operation conditions and dependenttherefrom providing control signals for controlling the selection andproduction of said submultiple lamp drive power signal.
 7. A method asdefined in claim 6, further comprising the step of:detecting theamplitude, phase and frequency of said resonant frequency of the saidresonant circuit, said amplitude being representative of a lampoperating condition.